Divvy is just one of many, many, many apps that brings Windows' Aero Snap-like functionality to your Mac, but it's arguably the best. Not only can you split windows up side by side, but you can do them in pretty much any configuration you want: left side and right side, top and bottom, two quarters and one half, and more. You can even assign keyboard shortcuts to your favorite window layouts, too, which is great. It used to be $14, but it's currently on sale for $6.99 if you buy it from the Mac App Store, so head over and grab it now if you haven't before. Windows users, sadly, still have to pay full price on Divvy's web site. Though frankly, it's still worth every penny.